A method for producing shaped bodies and associated apparatus are disclosed in BE-A-653,349 and SE-B-304,711 (both based on FR priority application No. 955,561 of 29 Nov. 1963). In this known method, an unhardened mixture comprising hydraulic cement and aggregate material (sand and gravel) with surplus water is compressed in an extruder of constant cross-sectional shape by means of a reciprocating piston, and in the terminal part of said extruder, the walls of which are suitably perforated, part of the water is removed by applying a vacuum to the outside of said walls, all this taking place while the material is moving slowly through the extruder.
Obviously, the pressure differential that can be produced by said vacuum arrangement is at the highest of the order of one bar. In addition to this, the reciprocating piston does, admittedly, exert a certain force, thus causing a corresponding increase in the pressure differential effecting the dewatering, but if sufficiently increased, this force will simply push the material out of the extruder, as no counter-force is provided to prevent this. This means, of course, that the total pressure differential across the perforated walls will at the most be of the order of a few bar. This in turn means that the ability of this previously known method to remove liquid from the spaces between the particles of the material is limited, and in many cases the quantity of the remaining liquid is sufficient: to prevent the shaped bodies produced from attaining more structural strength than just needed to keep their shape against the force of gravity, so that they, unless extreme care is taken, cannot be handled without deforming, collapsing or falling apart.
The above problem is, of course, less serious in the case of shaped bodies of clay, as such bodies can be allowed to or be made to harden respectively by well known methods before being moved, but the method referred to above is obviously insufficient, if the shaped bodies are to have a reasonable strength immediately upon having been produced by carrying out the method.